Pre-press proofing is a procedure that is used mainly by the printing industry for creating representative or intended images of requested printed material without the high cost and time that is required to actually produce printing plates. Time may also be critical for setting up a high speed, high volume printing press to produce an intended image. The intended image may require several corrections and be reproduced several times to satisfy or meet customers' requirements, resulting in a large loss of profits for the printer and higher cost for customers.
One such commercially available image processing apparatus is structured to form an intended image on a sheet of print media. A colorant is transferred from a sheet of donor material to a sheet of print media. The transfer is done by applying a sufficient amount of energy to the donor sheet material to form an intended image on the print media. The image processing apparatus generally includes a material supply assembly, a lathe bed scanning subsystem or write engine, which includes a lathe bed scanning frame, translation drive, linear drive motor, translation stage member, print-head, load roller, imaging drum, print media exit transport, and donor sheet material exit transport.
Operation of the above image processing apparatus includes metering a length of the print media (in roll form) from the material assembly. The print media is then cut into sheet form, of the required length, and transported to the imaging drum. Subsequently, the print media is wrapped around and secured onto the imaging drum. A load roller, which is also known as a squeegee roller, removes entrained air between the imaging drum and the print media. Next, a length of donor material (in roll form) is metered out of the material supply assembly or carousel, and cut into sheet form of the required length. The donor material is then transported to the imaging drum and wrapped around the periphery of the imaging drum. The load roller removes any air entrained between the imaging drum, print media, and the donor material. The donor material is now superimposed in the desired registration, with respect to the print media, which has already been mounted onto the imaging drum.
With the donor sheet material and print media secured to the periphery of the imaging drum, the scanning subsystem or write engine, provides the scanning function. This is accomplished by retaining the print media and the donor sheet material on the imaging drum while it is rotated past the print head. The translation drive axially traverses both the print head and translation stage member, along the axis of the imaging drum, in coordinated motion with the rotating imaging drum. These combined movements form an intended image onto the print media.
After the intended image has been formed on the print media, the donor sheet material is removed from the imaging drum without disturbing the print media beneath it. Next, the donor sheet material is transported out of the image processing apparatus to a waste bin. Additional donor sheet materials are sequentially superimposed with the print media on the imaging drum, further producing an intended image. With the completed intended image formed on the print media, the print media is removed from the imaging drum and transported to an external holding tray on the image processing apparatus.
Referring to FIG. 1, a prior art schematic of a rotational stop 230 for a linear drive motor 258, that is used in an image processing apparatus, is shown. The linear drive motor 258 is coupled to a threaded shaft 252 of a lead screw assembly 250 (shown, subsequently, in FIG. 3) Rotational stop 230 includes a stop button 261 that provides a single point of contact against a flat surface, where the rotational stop 230 is held by load magnet 286.
In prior art imaging apparatuses, linear error can occur when there is angular displacement of the linear drive motor 258 relative to the rotational stop 230. FIG. 2 shows linear error (ε) due to angular displacement (α) that is directly related to the location or distance of a stop button 261 relative to the linear drive motor 258. The greater the distance between the stop button 261 and the linear drive motor 258, the smaller the linear error (ε) will be. For some applications, however, it is impractical to place the stop button 261 at a great enough distance to have an acceptable small linear error (ε).
A person can see evidence of unacceptable linear error in an intended image by the amount of banding that is displayed in the intended image. Linear error due to angular displacement will cause the exposure distribution or density to be non-uniform. Banding in an intended image is a phenomenon that can be characterized as a periodic exposure density variation in an intended image. Conversely, a visually pleasing intended image should be uniform in exposure density. In general, the linear drive motor imparts rotation to the lead screw, which traverses the print head axially along the rotating imaging drum. As the print head traverses along the imaging drum an intended image is formed onto the print media, in the form of rows of halftone dots around the imaging drum. With each rotation of the imaging drum the print head is moved axially along the imaging drum and another row of halftone dots are formed onto the print media. One can easily understand that errors such as angular displacement could cause one row of half tone dots to be out of position relative to the next row of halftone dots, thereby causing banding in an intended image.
Although the presently known and utilized image processing apparatus is satisfactory, a need still exists to improve rotational error and reduce banding within the intended image.